The analysis of radiological images, and of biomedical images in general, is frequently based on the comparison of two or more images of the same patient and of the same anatomical district acquired at different times and/or with different modalities or acquisition protocols. Such a comparison is referred to as corresponding “views” among the two or more images under study.
The definition of “view” differs according to the type of image under study. In particular, in the case of 2d images, with “view” we refer to a subset of pixels of the original image, having an extension that may range from a single pixel to the entire image. Conversely, in the case of 3d images, with “view” we refer to a subset of voxels of the original image laying on a section that can be either axial, coronal, sagittal or oblique, and the extension may range from a single voxel to the entire section.
Once a view VA has been identified in an image A, the manual search of the corresponding view in B, VB, is a time consuming operation. This is described for instance in document US2007230761 (A1).
Automatic or semi-automatic registration techniques allow accelerating this phase, as they (semi-) automatically identify the view VB in image B corresponding to VA.
Registration can be either rigid (that is, based on an affine transformation) or non-rigid. The non-rigid registration is necessary for images that are not completely overlapping due to changes in the anatomy caused, for instance, by patient motion during acquisition in the case of dynamic images, or by tumor evolution over time in the case of images acquired at different times.
Once the registration R is completed, some visualization systems visualize the two views VA and VB without any further elaboration, while others apply to VB the transformation TR corresponding to the registration R. In that latter case the visualization refers to VA and TR(VB). It is important to notice that, in the case of a non-rigid registration, it may happen that a view VA corresponds to a view VB in B. which lies on a non-planar surface, and the 2d visualization is thus not possible unless some transformation TR or some other deformation (such as a projection) is applied to VB.
Once the registration R is completed and the eventual transformation TR is applied, given a view VA in A, the corresponding view VB in B can be visualized either side-by-side (this type of visualization is typically referred to as “parallel” or “synchronized” visualization, as the choice of visualization for A automatically induces the visualization choice for B) or overlaid to VA (this type of visualization is typically referred to as “fusion”), with transparence level settable by the user, in which the second view can be either VB itself or the transformed view TR(VB).
Both visualization modes, overlaid or parallel, have a set of advantages and drawbacks that make the user prefer one mode or the other depending on the particular context.
In particular the overlaid visualization has the following advantages:
Point-wise correspondence, for which it might, however, be necessary to resort to a non-rigid transformation;
The corresponding elements of the two or more images are in the same field of view, as they are exactly overlapping, and it is thus not necessary that the user change its gaze in order to compare the two views.
At the same time the overlaid visualization has the following disadvantages:
Low reliability for diagnostic purposes, pre-surgical planning or intra-operative guidance in the case of registration different from a purely affine transformation; as already mentioned, in the case of not perfectly overlapping views, in order to overlap view VB to view VA, the former must be transformed according to the registration; view VB is thus deformed with respect to the acquisition and may no longer reflect the original shape and dimensions; as such it cannot be used for taking any measure for diagnostic purposes, pre-surgical planning or intra-operative guidance; in general, even for a qualitative analysis, image deformation has obvious consequences on the diagnostic reliability;
Excess of information in the same point, in the case of three or more images; in such a case, in fact, without any further elaboration of the original images, by fusing three or more views, in the same point an excess of information is obtained, which is not easy to interpret and is thus not useful for diagnosis, pre-surgical planning or intra-operative guidance.
The parallel visualization mode, instead, has the following advantages:
It is effective, without necessarily having to resort to non-rigid transformation; for instance, for a transformation composed of a rotation, a translation and a scaling factor along a certain direction, it is possibly to apply to B only the rotation and translation thus avoiding the deformation caused by the scaling factor;
It can be applied also to three or more images, if the screen dimension and resolution allow it.
At the same time the parallel visualization mode, instead, has the following disadvantages:
The correspondence is defined over entire regions and it is not point-wise, as it is based on global rigid transformation; this means that the region in its complexity is indeed registered correctly, while a single anatomical element within the region may appear translated, shrunk or even disappear in one or more views of the images other than A;
In order to compare the same anatomical point on two or more different images, the operator necessarily has to change his/her field of view, thus reducing the overall efficiency of the analysis; this limitation was already faced in similar contexts: for instance in US2010201708(A1), the attention is brought on the problem that the operator is distracted by the need to change the eye fixation (the mentioned patent is related to a single image which is the result of the fusion of two images and the problem is related to the fact that the operator has to change the gaze from the fused image to a menu bar in order to be able to change the images which are fused together);
Impossibility of visualizing side-by-side original images in the case of non-rigid registration; in fact, it may happen that a planar view VA corresponds to a non-planar view VB in B, so that the visualization of the non-planar view VB is impossible unless some further deformation is applied (such as a projection).